An electrochemical battery such as a lithium ion secondary battery is provided between the positive electrode and the negative electrode with a separator which electrically insulates these electrodes and holds electrolyte. Such separators, particularly those in lithium ion secondary batteries, are currently made of microporous film mainly made of polyethylene, polypropylene or the like.
However, in general, separators made of these materials shrink easily at high temperatures. If the battery shorts out internally or a sharp-pointed object such as a nail has penetrated through it, there is a sudden generation of reaction heat of the short circuit. As a result, the separator shrinks to increase the short circuit area so as to generate a larger amount of reaction heat. This repeated alternation between the separator shrinkage and the generation of the large amount of reaction heat causes an abnormal rise in the temperature inside the battery.
In order to increase the battery safety, it has been proposed to form a porous coating film containing solid microparticles onto the surface of an active material layer (See Patent Document 1).
[Patent Document 1] Japanese Patent No. 3371301